Feature: Gather all signing code in signer layer

[logicalangel]

As you know, We use several signing algorithms in Unchained, and different services use these signing methods. These points are important to consider:

• We need to prevent duplicate codes and have just one function for signing and verifying data.
• Services should not depend directly on signing methods. signing data is not the service's responsibility.
• We know that we are going to add new signing methods, so changing signing methods should not lead to changing other layer's codes.

So:

• we need to gather all signers and their functions behind an interface.
• we want to add the TSS algorithm as a new signing method.
• we need to change the dependency of signing in services to interface.